The prior patent discloses a blood filter and method of filtration for removal of leucocytes from whole blood or blood fractions. The filter material is a shape-sustaining laid textile fiber web having a thickness of at least about 1 millimeter and a bulk density of between about 0.05 and 0.4 g/cm.sup.3. The web has a plurality of interlocked, textile fibers which are substantially uniformly distributed in the web so as to form a matrix of the fibers with spaces between interstices of interlocked fibers. Within those spaces are disposed a plurality of fibrillated particles of polymeric material having a plurality of fine fibrils which are interlocked with the adjacent textile matrix fibers such that the fibrillated particles are not substantially displaceable from the web during filtration of blood, and the textile matrix fibers form a support for the web and the fibrillated particles.
As disclosed in that prior patent, for efficient and effective depletion of leucocytes from blood passing through the filter material, both the fiber geometry and the surface area of the fibers are important, and that, very importantly, the surface area must be significantly greater than the usual prior art blood filters, since otherwise the degree of leucocyte depletion (removal) is not sufficient. Further, since fiber geometry and surface area are important for leucocyte depletion, the depth (thickness) of the filter material is also important. Somewhat similarly, since the bulk density of the filter material and the denier of fibers affects fiber geometry and surface area, these are also important.
However, to achieve the high surface area of the filter material required for effective leucocyte removal from blood, a critical component is that of the very high surface area fibrillated particles in the filter material. Ordinary textile fibers cannot provide such high surface areas to the filter material which surface area is required for high leucocyte removal. The fibrillated particles are somewhat elongated particles with an elongated central portion from which radiate a large number of very fine fibrils.
As can be appreciated, it is important to ensure that the small fibrillated particles are not significantly displaced from the filter material during filtration of blood, and, as disclosed in that prior patent, this is achieved by interlocking the fibrils of the fibrillated particles with adjacent textile matrix fibers. Further, especially at higher weight ratios of fibrillated particles to textile matrix fibers, e.g. 6:94 to 10:90, permanent securing of the fibrillated particles in the filter material may be improved by use of means for adhering the textile matrix fibers and fibrillated particles to each other, such as by tackifying adhesives and especially the use of sheath/core fibers for at least part of the textile matrix fibers, e.g. a sheath of low melt temperature polymer and a core of higher melt temperature polymer. When the textile matrix fibers are at least in part such sheath/core fibers, during usual processing of the filter material web, including heating thereof, the sheath softens and causes bonding, upon cooling, between the textile matrix fibers themselves and fibrillated particles, especially the fibrils thereof.
While the filter materials of that prior patent are quite effective for leucocyte removal from blood or blood fractions, the above-noted prior application discloses that the overall performance of those filter materials may be improved in that the efficiency of the filter material (percent leucocyte removal per unit thickness of filter material) can be increased. This means that for a targeted leucocyte removal percentage, the improved filter material of the prior application can be of less thickness than the filter material of the prior patent. This, in turn, means that the amount of blood or blood fractions retained in the filter material of the prior application, after filtration is completed, is less than that of the filter material of the prior patent. While this difference in the amount of retained blood is not an absolute large amount, the difference can be quite important, especially in certain filtrations of blood.
Further, with the filter material of the prior application, less fibers are released from the filter during an AAMI test than with the filter materials of the prior patent.
In regard to the prior application, it was found that if the average sizes (diameter and length) of the matrix fibers are, generally speaking, less than the average sizes of the matrix fibers of the prior patent, increased efficiency of the filter material occurs. It was also found that this is especially true when the matrix fibers are, at least in part, made of certain inorganic materials, especially glass. These smaller matrix fibers could nonetheless be adequately interlock with the fibrillated particles to prevent displacement of either the matrix fibers or fibrillated particles from the filter material during blood filtration when the filter material has therein an added thermo-softening binder.
With such binders, fibrillated particles of larger surface area may also be used and the efficiency of the filter material (and hence the required thickness of the filter material for a targeted leucocyte removal) could further be reduced.
The prior application discloses that the textile matrix fibers of the prior patent may be replaced, at least in part, by the smaller diameter glass matrix fibers when the glass fibers have a diameter between about 0.1 and 5 microns, and especially when the diameter is between about 0.5 and 1 micron, most preferably about 0.65 micron. Example 2 of that prior application shows a 99.9% reduction in leucocytes when the filter material uses 85% of 0.65 micron diameter glass fibers, and the blood being filtered is a usual concentration of blood cells. However, it has been found that, when the concentration of blood cells in the blood to be filtered is increased to higher levels, that percent reduction in leucocyte content of the filtered blood very significantly drops.
Thus, while the filter of the prior application is most satisfactory for the more usual blood filtrations, it is not entirely satisfactory for a wider range of blood filtrations, especially in regard to blood having higher concentrations of blood cells.
It would, therefore, be of substantial advantage to the art to provide an improved filter which can achieve high percentage of leucocyte depletion over a wide range of blood filtrations, especially when the blood filtration involves higher levels of blood cells.